Capacitive micromachined ultrasonic transducers (CMUTs) are electrostatic actuators/transducers, which are widely used in various applications. Ultrasonic transducers can operate in a variety of media including liquids, solids and gas. Ultrasonic transducers are commonly used for medical imaging for diagnostics and therapy, biochemical imaging, non-destructive evaluation of materials, sonar, communication, proximity sensors, gas flow measurements, in-situ process monitoring, acoustic microscopy, underwater sensing and imaging, and numerous other practical applications. A typical structure of a CMUT is a parallel plate capacitor with a rigid bottom electrode and a movable top electrode residing on or within a flexible membrane, which is used to transmit (TX) or receive/detect (RX) an acoustic wave in an adjacent medium. A direct current (DC) bias voltage may be applied between the electrodes to deflect the membrane to an optimum position for CMUT operation, usually with the goal of maximizing sensitivity and bandwidth. During transmission an alternating current (AC) signal is applied to the transducer. The alternating electrostatic force between the top electrode and the bottom electrode actuates the membrane in order to deliver acoustic energy into the medium surrounding the CMUT. During reception an impinging acoustic wave causes the membrane to vibrate, thus altering the capacitance between the two electrodes.
Because the electrostatic force in the CMUT is nonlinear, then as the separation space between the two electrodes decreases during actuation, the electrostatic force between the electrodes typically increases at a greater rate than a restorative force of the membrane. Therefore, when the movable electrode displaces to a certain position, e.g., typically one-third of the electrode gap, the restorative force of the membrane is not able to balance the electrostatic force. Any further voltage increase can cause a “pull-in” effect that can result in instability or collapse of the CMUT. Consequently, in order to achieve enough displacement for certain applications, the separation gap between the two electrodes has to be designed to be much larger than the displacement actually required, which can fundamentally limit performance of CMUTs in a conventional operation.